First, let's look at phonetic similarities and differences among
some of the oldest representatives of the main branches of the
Indo-European languages. The letters in red are ones that
differ from the original Proto-Indo-European:

PIE

Sanskrit

OCS

Lithuanian

Armenian

Greek

Latin

OIR

Gothic

Hittite

Tokharian*

P

P

P

P

H, -W

P

P

---

F, -B

P

P

B

B

B

B

P

B

B

B, M,
-W-

P

P

P

Bh

Bh

B

B

B, -W-

Ph

F-. B

B, M, -W-

B

P

P

T

T

T

T

T

T

T

T.
-Th

TH. -D

T

T, C

D

D

D

D

T

D

D

D, -Dh

T

T

T, ---, S'

Dh

Dh

D

D

D

Th

F, D, B

D, -Dh

D

T

T, C

K'

S'

S

S'

S

K

K

K

H, -G

K

K, S'

G'

J

Z

Z'

C

G

G

G, -Gh

K

K

K,
S'

G'h

H

Z

Z'

J, Z

Kh

G

G, -Gh

G

K

K,
S'

K

K, G

K, C',
C

K

K'

K

K

K, -Kh

H, -G

K

K, S'

G

G, J

G, Z',
DZ

G

K

G

G

G, -Gh

K

K

K,
S'

Gh

Gh, H

G, Z', DZ

G

G, J

Kh

H, G

G, -Gh

G

K

K,
S'

Kw

K, C

K, C', C

K

K'

K, P, T

Kw, K

Kw, K

Hw

Kw

Kw, K

Gw

G, J

G, Z', DZ

G

K

B, D, G

Gw, G, W

B, M, -W-

Kw, K

Kw

K, Kw

Ghw

Gh, H

G, Z', DZ

G

G, J

Ph, Th, Kh

Gw,G,W,F

Gw

Gw, G, W

Kw

K, Kw

S

S, S'

S

S

S, H, ---

S, H, ---

S, R

S, ---

S

S

S

R

R

R

R

R

R

R

R

R

R

R

L

R, L

R, L

L

L

L

L

L

L

L

L, Ly

M

M

M

M

M

M

M

B, M, -W-

M

M

M

N

N

N

N

N

N

N

N

N

N

N, Ny

Y

Y

Y

Y

---

Y, ---

Y

---, Y

Y

Z, H, ---

Y

W

V, W

V

V

G, V

W, H, ---

W

F, ---, W

W

W

W, Y

PIE

Sanskrit

OCS

Lithuanian

Armenian

Greek

Latin

OIR

Gothic

Hittite

Tokharian*

(Chart based on Christopher Gwinn's "Indo European Phonology,"
http://tied.narod.ru/)

* Note: The column on Proto-Tokharian was added by myself.
Although it begins as a Centum language, most consonants were
later palatalized before front vowels. This happened with some
other Centum languages as well.

While S, R, L, M, N, Y, and W do not change that often, the
plosives often change dramatically.

One basic set of differences is that between the Satem languages - Sanskrit,
Old Church Slavonic (OCS), Lithuanian, and Armenian - and the Centum languages - Greek,
Latin, Old Irish (OIR), Gothic, Hittite, and Tokharian.

In Centum languages, PIE palatalized velars k', g', and g'h merge
with k, g, and gh, while bilabialized velars kw, gw, and ghw
retain their identity (up to a point!). In Satem languages, PIE k'
becomes s or sh, g' becomes z or zh, g'h becomes z or zh, while
kw, gw, and ghw merge with k, g, and gh, with some additional
palatization occuring later in their development. Notice how
Armenian loses the voiced-unvoiced distinction among the plosives,
and Hittite and Tokharian lose that as well as the distinction
between plain and aspirated plosives.

The Indo-European languages of the Tarim Basin in far western
China known as Tokharian have some Celtic sound qualities -
leading some to suggest that a branch of early proto-Celts
wandered all the way to China. Another scenario is a little
more sensible: The ancestors of The Tokharians left the
eastern side of the main PIE area at about the same time as the
Celts and other western Centum languages left the western side,
perhaps a little earlier, and so prior to the same changes in the
Balto-Slavic and Indo-Iranian dialects that the Celts missed.

In Greek , PIE bh, dh, and gh become phi, theta, and chi, i.e.
first become unvoiced aspirated stops and eventually unvoiced
fricatives. Note that in Latin, bh, dh, and gh become f, f,
and h, respectively. I suspect that the progression was to
ph, th, kh, then to phi, theta, chi, and finally to f, f, and
h. In most other languages, bh, dh, and gh become b, d, and
g (or z/zh, if satem), respectively. Also, Greek collapses
PIE kw, gw, and gwh somewhat indiscriminately into k/p/t, b/d/g,
and phi/theta/chi.

In Gothic we see some of the most dramatic (although systematic)
changes: PIE p, t, k, and kw become f, th, h, and hw, while
b, d, g, and gw become p, t, k, and kw.

In OIR, PIE b, gw, bh, and m all become b or m or
w (depending on context). Note also that all plosives become
aspirated at the ends of words, and eventually become fricatives.

Note: Languages related to Latin, as well as ones related
to OIR, sometimes use p in place of kw. This is another
commonality, then, between the Italic family and the Celtic
family.

What groupings might we tentatively make on the basis of these
comparisons?

Unfortunately for the elegance of the preceding, other approaches
give us other results. If we look at certain grammatical
constructions, for example, we find the following:

For the genitive singular of nouns, we have Greek, Armenian, and
Indo-Iranian (Sanskrit) using -osyo, Slavic (OCS) and Baltic
(Lithuanian) using -ó, Baltic and Germanic (Gothic) using -eso,
and Celtic (OIR) and Italic (Latin) using -í.

For indirect and dative cases, we find a similar pattern:
Greek, Armenian and Indo-Iranian using -bhi, Slavic, Baltic,
Germanic, and Tokharian use -m . Hittite uses -n. Celtic and
Italic using -bhos (dative singular)

The Celtic-Italic link is fortified by such constructions as the
comparison in -samo (vs -tero, -isto) and medium voice in -r (vs
-oi, -moi).

The Greek-Armenian-Indo-Iranian link is fortified by the fact
that all three have an athematic and a thematic aorist.

Hittite differs in many ways from the others, although showing
more similarity to Celtic-Italic than to other groups.

So, while Celtic-Italic remain independent, Greek and Germanic
have jumped the Centum ship, you might say, to join the two Satem
sub-groups - or more likely, the reverse: left the central dialect
area (Slavic-Baltic and Armenian-Indo-Iranian) prior to the Satem
innovation, but well after Celtic-Italic left, and even further
after the Hittite and Tokharian left.

Lexicon

And finally, there is the issue of common words. Analyses
seem to show that Hittite and Tokharian split from the mainstream
earliest. Later, the Celtic and Italic languages split off,
followed by Greek and proto-Armenian. Albanian also splits
off somewhere in the same time frame. Finally, we have the
Indo-Iranian and Germanic languages splitting off from the central
mass of Indo-European, which eventually becomes Balto-Slavic.

Lexicon analysis generally supports the morphemic hypotheses, and
is not too contrary to the phonetic hypotheses.

A sticking point is the place of Germanic: It seems to be
both clearly tied to Baltic, yet phonetically quite
distinct. It is, in fact, quite distinct from all its
relations! One theory is that its similarity to Baltic is in
part due to close proximity and continued contact, and that its
phonetic distinctiveness is due to a pre-Germanic substrate of
megalithic people sometimes referred to as "the Folk."

It is possible that pre-Indo-European neolithic people in France
and England likewise influenced the development of the Celtic
languages as well - another "Folk" perhaps. The neolithic
people of the Iberian peninsula and southwestern France -
including the ancestors of the modern Basques - do not appear to
have had too significant an effect on the Celtic of that area, nor
on the later Latin.

Other languages have had more obvious contact with other language
substrates. The Hittites and other Anatolians clearly came
into an area already well-populated with non-Indo-Europeans, which
no doubt hastened the rather dramatic phonetic and morphemic
simplifications characteristic of those languages.

The Mycenaean Greeks may have been influenced by an Anatolian
substrate in Greece and the islands (the Pelasgians?), further
complicated by a second wave of Greeks (Dorians).

The Armenians appear to be closest to the Indo-Iranians in
phonetic and morphemic structure, and were possibly influenced by
centuries of contact with them as well as the Hittites, Semites,
and Caucasians.

An Evolutionary Time-line of the Indo-European Languages

c. 5000 bc.

Homeland: The Danube River valley (Wallachia and
Hungary). Farming learned from the people of Asia
Minor. Cultivation of native rye and oats and domestication
of native pigs, geese, and cattle begins. Strong tribal
sociey develops. This is the hypothesis first presented by Igor M. Diakonov. Note: Most linguists follow Marija Gimbates' Kurgan theory. A smaller group follow Colin Renfrew's Anatolian theory. I am admittedly only an amateur, but I prefer Diakonov's Balkan theory, which is also a part of Renfrew's extended theory.

There are many reasons for choosing the Danube River
valley: Farming is possible, although the land is less
than desirable to more powerful tribes from the south; the flora
and fauna of the valley, as well as for other natural features
such as hills and rivers, are represented by the oldest words we
can reconstruct; it includes the natural ranges of wild horses
which, when later domesticated, would become the Indo-European's
"ace card"; the area is central to the eventual expanse of
the Indo-Europeans, with due allowance for the more rapid
expanse commonplace over steppe-lands; the area is also in
close proximity to some of the most conservative recent
representatives of the family.

The most compelling reason is the presence of the Danubian
culture, with its linear incised pottery, at this same
time. The culture spreads soon after in exactly the
directions that would account for the spread of PIE.

There are, of course, many other possibilities. The most
common suggestion is the steppes north of the Black Sea, for
many similar reasons. I believe that the strong tribal
social structure suggests that the Indo-Europeans were farmers
before they were pastoralists. It is highly unlikely that
they went straight from steppe hunter-gatherers to sophisticated
pastoralists in one step.

c. 4000 bc.

Proto-Anatolians move east to the northern
Caucasus. They would be profoundly influenced by the
advanced cultures of Asia Minor and beyond.

Proto-Tokharians move east into the Ukraine. These
people are the most likely originators of the horse
culture. There is also plenty of evidence of ox-drawn
wagons with disk wheels in the western steppes.

A western dialect emerges on the upper Danube and beyond.
The enclosed steppe of the Hungarian Plain is an ideal position
to blend farming with a horse culture.

c. 3000 bc.

Copper working, learned from the people of Asia Minor,
begins in Thrace and the Danube valley and reaches Germany by 3000
bc.

Domestication of the horse spreads from the Ukraine.
Within a thousand years, horsemanship spreads from the Ukraine
throughout the Indo-European area, even into Scandinavia.
It is the steppe inhabitants who change most dramatically into
true pastoral societies. In the more wooded areas of
Europe, horse ownership begins to differentiate a warrior
nobility from commoners. Of course, use of the horse
spreads to the non-IE societies of the Middle East as well.

The disk-wheel wagon has spread from Russia across Europe to
Holland.

The Proto-Anatolians move from the Caucasus to Asia
Minor.

The Proto-Tokharians continue east to the steppes, towards the
Tarim Basin in northwestern China. They may be the people
known to the Chinese as the Yüeh-chi, and may have been the core
of the Kushan Empire of the first century AD.

The Proto-Celts separate from the rest of the western dialect
and expand west into southern Germany and France, where they
develop the Michelsburg culture and begin to strongly pressure
the pre-PIE people, likely including the ancestors of the
Basques and Aquitanians. The remaining western dialect
tribes edge into the modern Slovenia-Croatia area as well as
northern Germany.

The main body of Indo-Europeans expands into Thrace, the
Ukraine, Bohemia, and Poland, and begins to differentiate into a
northern dialect (Bohemia, Poland, and Hungary, represented by
the Funnel Beaker culture) and a southern dialect (Wallachia,
Thrace, and Ukraine, continuing the Danubian culture). The
original inhabitants north and west of the Carpathians, likely
speakers of Uralic languages, are pushed further north and east.

c. 2500 bc.

Bronze working develops throughout Indo-European area.

The Proto-Italics, who speak a western dialect, move
west and south from the Slovenia area into Italy. There they would
encounter well-established pre-PIE people, possibly the ancestors
of the Etruscans and Rhaetians.

The Proto-Illyrians, speaking a western dialect (perhaps), move
south from the northern Croatia area into Illyria (the Dalmatian
coast).

One branch of the southern dialect - Proto-Hellenic - moves
south into Macedonia, Greece, and the Aegean islands, absorbing
much of the Pelasgian people and culture. By 1500 bc, the
southern-most tribes would establish the Mycenaean culture.

The Proto-Germanics move into Scandinavia. Odd aspects of
Proto-Germanic may be due to interaction with northern Celtic
tribes, Baltic tribes, and possibly to the presence of native
speakers of Uralic languages in Scandinavia.

The remaining main body of Indo-Europeans (the Baltic, Poland,
Bohemia, the Hungarian Plain, Wallachia, Thrace, the Ukraine and
the neighboring steppes) - both northern and southern dialects -
undergoes the Satem phonetic changes.

c. 2000 bc.

The horse-drawn, two-wheeled chariot, with spoked
wheels, is developed in the western steppes, and spreads quickly
to the Balkans as well as the Middle East.

A branch of the southern Satem dialect - Proto-Indo-Iranian -
expands from Ukraine and the steppes into Afghanistan, Iran, and
into India. One tribe - the Mittani - goes as far west as
northern Mesopotamia. The well-established cultures influence
the newcomers greatly, but the Proto-Indo-Iranians maintain
their language.

The main body of the southern Satem dialect expands into the
Ukraine to become the Cimmerians, leaving the Dacians in the
original homeland. I suspect that the Dacians were a
southern (Cimmerian-like) dialect. The people of Thrace
were probably closely related to the Cimmerians, also with a
southern Satem dialect. These people develop the steppe
version of the Battle Ax culture.

The main body of the northern Satem dialect -
Proto-Balto-Slavic - expands north from Poland into Belarus and
the Baltic coast. With the Germans, they consititute the
northern version of the Battle Ax culture.

The Celts expand further into western Europe and, in a
retrograde move, back into Hungary. A powerful society,
they pressure the original peoples of western Europe, as well as
their own relations to the east. They develop the
Bell-beaker culture and, later, the Urnfield culture.

Anatolians (most notably the Hittites) establish themselves in
Asia Minor, where they become a major power. Their
languages are profoundly affected by neighboring non-IE
languages.

A second wave of Hellenics (Doric Greeks) moves into Greece
from Macedonia.

c. 1500 bc.

Proto-Phrygians - possibly a branch of the Cimmerians -
move from Thrace across the Bosporus to northwestern Asia
Minor. The Phrygians would move into the power gap left by
the collapse of the Hittite Empire around 1200 bc.

Proto-Armenians - possibly another branch of the Cimmerians -
move into Asia Minor, probably by means of the Bosporus.
It is possible that they entered from the east coast of the
Black Sea, or even across the Black Sea. In the next 1000
years, they spread over much of northern Asia Minor, but are
eventually pressured into the Lake Van region.

Albanian may be the sole survivor of the Illyrian languages,
its many variant features due to long contact with a variety of
neighbors. Or it could be the lone descendent of a Dacian
dialect that later moved into the Albanian region. I lean
toward the latter, but it is very difficult to tell! So
little of Dacian, Thracian, Illyrian, etc. is left to us.

The western Celts expand into western Iberia and the British
Isles, where they absorb most of the prior inhabitants.
The original inhabitants of Spain survive well into the Roman
era, while the original inhabitants of southwest France survive
to the present as the Basques.

The Balto-Slavics differentiate into Baltic and Slavic.
Both begin to expand east- and northward, at the expense of the
hunter-gatherer Finno-Ugric people.

The Indo-Iranians differentiate into Indic and Iranian. The
Indic group rapidly expands across northern India as far as
Magatha. The Iranians split into powerful tribes, notably
the Persians and the Medes, by the 800's bc. The Iranians
remaining in the steppes would come to be known as the Scyths
and Sakas. The powerful Scyths expand westward at the
expense of the remaining Cimmerians.

c. 1000 bc.

Iron working begins in the Balkans by 1000 bc. It
reaches Britain by 800 bc.

The "Age of Empires" begins in earnest.

Major resources: I. M. Diakonov, On
the Original Home of the Speakers of Indo-European, Journal of
Indo-European Studies (1985, vol. 13, pp 92 to 174) and J. P.
Mallory's In Search of the Indo-Europeans (1989). Maps
based in part on Colin McEvedy's The Penguin Atlas of Ancient
History (1967).